All split keyboards I’m aware of connect the halves with 3.5mm TRRS jacks, if not wireless. QMK and ZMK boards alike. Why is that?
They provide the required number of wires, the jacks are readily available and cheap, the cables are readily available in various lengths, the cables are flexible, and look classier than RJ9.
Yeah they are a common’ish piece of equipment which sure helps.
I wonder what kind of protocol they are using on the wire?
Zmk uses i2c - I would guess qmk as well, but I wouldn’t know
I just got a Keychron Q11 which uses usb-c to connect the halves. I think I preferred the TRRS on my Moonlander for the aesthetics.
Interesting! Yeah USB-C would seems like the main alternative. The cables are ubiquitous and the board anyway has some USB capabilities for output. It doesn’t seem like a stretch to use it for halfwise communication as well.
Maybe the USB protocol has just been seen as an overkill for this use?
The most common development board for keyboards at the time was the arduino pro micro, which didn’t support more than one USB connection at a time - most development boards don’t support that until now, so to use the USB protocol, you’d have to use two development boards per side, which makes everything expensive and complicated.
Due to the damage risk of TRRS, a few modern builds use usb-c connectors - but only for the electrical connection: underneath it’s still the same 4-wire protocol.
The first split keyboard with qmk (the “Let’s split”) was announced 2014 and was in development a lot longer before, while usb-c was only announced 2014, so even for just the 4-wire communication that connector would’ve been too late.
And I think the “Let’s split”'s influence was strong enough for the TRRS to keep being used despite their risk of frying your chipset.
